Wandering around the prison site on Capraia, we found a rubbish dump with some
pieces of broken glass bottles. We were amazed to find that when placed over the hole in
the top face of the box, the bottle bottoms actually acted as lenses. You
could experiment with different lenses; try spectacles or a magnifying glass.

bottle-bottom lens with rotating mirror in its housing

The screen is just a piece of tracing paper mounted on a moveable wooden frame. To
focus the camera, the screen is moved up and down (in other words, nearer
to and further away from the lens) until the image is as sharp as you can
get it. Of course, it helps if the subject to be photographed is as still
as possible.

Some light passes through the screen onto the lower mirror, which allows you to look
at the image from a comfortable angle through the open side of the camera.
This open side should be covered with two or three black bin-liner bags
to keep the light out and to allow the operator in.